The present invention relates to a dielectric filter which may be used as a microwave filter, for instance, and in which an internal conductor and an external conductor, each formed of dielectric material, resonate.
A surface mount type dielectric filter used so far as a microwave filter comprises a cuboidal dielectric block of dielectric material, through which one or more holes are formed. One or more through-holes are provided with internal conductors on their internal surfaces while the dielectric block is provided on its external surface with an external conductor acting as an earth electrode. The internal conductors are electrically separated from the earth electrode by shaving off a part of an electrode on one end face of the dielectric block, from which the one or more holes extends therethrough. A filter with a hole is called a coaxial filter, and a filter with more than one holes is called a comb line filter. The coaxial filter comprises providing inner conductors on inner walls of resonators of the coaxial type, respectively, which are dielectric, locating the resonators between terminals, and connecting the inner conductors in series to the terminals through capacitors (See U.S. Pat. No. 4,799,033, FIG. 5 and FIG. 6). The comb line filter comprises providing holes in a cuboidal dielectric block of dielectric material along a center line thereof with an equal interval interposed between them, forming inner conductors of plating on inner walls of the holes, providing conductors on those faces of the dielectric except the top face thereof at which the holes are opened, and connecting the inner conductors located on both sides to terminals through capacitors (See U.S. Pat. No. 4,799,033, FIG. 7 and FIG. 8). An external coupling to the circuit board is made by forming a signal input-output terminal on the external surface of the dielectric block (See U.S. Pat. No. 4,879,533, FIG. 1). In many cases, the input-output terminal is made up of a metal lead.
However, the conventional dielectric filter including a metal lead form of input-output terminal is found to have the following problems.
(1) The input-output terminal has a low tensile strength. PA1 (2) Upon excessive heat applied to the dielectric filter mounted on a circuit board, there is often a variation of the filter characteristics due to the fusing of solder by which the input-output terminal is fixed in place. PA1 (3) Some considerable expense is needed because of not a few steps needed to mount the input-output terminal on the dielectric filter and a cost of the input-output terminal per se. PA1 (1) To prevent chipping of the dielectric block, the edges of the dielectric block should be chamfered during its formation. Due to the presence of such chamfered edge areas, however, it is difficult to carry out printing for the formation of the input-output terminal electrode. PA1 (2) When the dielectric block is obtained by axial-direction pressing, the green density of the dielectric block decreases substantially at its central area. Consequently, the dielectric block contracts more largely in the vicinity of its central area due to a shrinkage upon firing; the surface mount face of the filter fails to provide any flat plane. It is thus difficult to carry out printing for the formation of the input-output terminal electrode. PA1 (3) The face of the dielectric block on which the input-output terminal electrode is to be mounted is unstable. This makes the electrode area of the input-output terminal susceptible to variations. Consequently, the input-output capacity value of the dielectric filter varies with a variation of the filter characteristics.
As known in the art, such problems as mentioned above may be solved by use of a dielectric filter comprising an input-output terminal electrode formed on the surface mounting face of the dielectric filter (See U.S. Pat. No. 4,879,533, FIG. 4A).
In a dielectric filter comprising an input-output terminal electrode formed on the surface mounting face of the dielectric filter, the external conductor and input-output terminal electrode are formed in the following manner. Ceramic material powders are first pressed and fired to obtain a dielectric block. Then, one portion--to be provided thereon with an input-output terminal electrode--of one face of the dielectric block which provides a surface mount face is dipped in an electrically conductive coating material while masked according to an input-output terminal electrode pattern, followed by firing. Subsequently, an electrically conductive coating material is screen printed on the portion masked as mentioned above according to a pattern depending on the required purpose of the filter, and then fired to form an input-output terminal electrode that is an electrode for external coupling purposes. Finally, while the electrical properties of the thus fabricated dielectric filter are measured, a part of the electrode is shaved off for the regulation of resonance frequency. However, this process is found to have the following problems.
In view of the aforesaid problems, it is an object of the present invention to provide an inexpensive dielectric filter which has uniform filter characteristics, and is fabricated through a reduced number of steps with no need of any regulation operation, and a fabrication method thereof.